1. Field of the Invention
The present invention relates to a memory unit having a characteristic correcting function and to a semiconductor device to which the memory unit is mounted.
2. Description of the Related Art
A semiconductor memory (abbreviated to a memory, hereinafter) is a typical memory unit which is configured mainly by a MOS transistor, and has advantages in that it is capable of operating at a higher speed than a magnetic recording medium, does not require a mechanical driving part, few breakdowns occur, the lifetime is long and the like. Included in the typical memories are a DRAM (Dynamic Random Access Memory) and an SRAM which are volatile memories, and a masked ROM, an EPROM, an EEPROM, a flash memory, and a ferroelectric memory which are nonvolatile memories and the like. These memories each of which is formed using a single crystalline semiconductor substrate have already been put into practice and abundantly available in the market. Recently, a technology for monolithically forming an integrated circuit and a pixel portion onto a glass substrate, namely a system-on-panel has been expected to be advanced in a flat panel display which is formed by using an inexpensive glass substrate. Accompanied by this, a study of the practical application of a memory using a thin semiconductor film has been promoted.
There are various kinds of memories as described above and in each memory, generally, a memory cell is disposed at an intersection area of a word line and a bit line. Provided in the memory cell are a memory element and, in many cases, a switching element for controlling writing data into the memory element. By selecting certain word line and bit line, each memory cell is selected and writing and reading of data can be performed.